DESCRIPTION: (Applicant's Abstract) Glutamate transporters perform an essential function at excitatory synapses in the brain by helping to terminate excitatory signals. Based on their molecular diversity, distinct patterns of expression and propensity for modulation, it seems likely that glutamate transporters play additional, more active roles in excitatory synaptic function. However, their potential contributions to excitatory signaling remain unexplored. The broad objectives of our research are to understand glutamate transporter function at intact synapses and to define roles for transporters in excitatory signaling. This proposal uses whole-cell electrophysiological techniques applied to brain slices to focus on glutamate transporters at the parallel fiber (PF) to Purkinje neuron (PN) synapse in the cerebellum. PF synapses are regulated by a form of activity-dependent plasticity which is triggered by activation of a G- protein coupled glutamate receptor (mGluR1a). This receptor type is colocalized with a postsynaptic glutamate transporter present on PN dendrites. The close arrangement of these two molecules raises the possibility that glutamate transport controls mGluRla activity by limiting extracellular glutamate concentration at PF synapses. Each of the aims in the proposal specifically addresses a component of this hypothesis. The first aim examines the degree to which postsynaptic glutamate transporters limit glutamate concentration during PF synaptic activity. The second aim examines the effects of blocking glutamate transport and of manipulating glutamate concentration on the synaptic activation of mGluRla. The third aim tests if mGluRla modulates glutamate transport, a hypothesized negative feedback loop expected to regulate synaptic plasticity at PF synapses. These experiments illuminate a novel mechanism for controlling experience-dependent changes in excitatory circuitry in the brain. The findings should also lead to a better understanding of glutamate synapses and provide insight into diseases associated with malfunctions in excitatory transmission such as epilepsy and Amyotrophic Lateral Sclerosis.